Animals with a Backbone
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The vertebrates comprise a large group of chordates, and are subdivided into seven classes (3 classes of fish, amphibians, reptiles, birds, and mammals). Vertebrates have an internal skeleton of cartilage or bone, with vertebrae surrounding the dorsal nerve cord.
The subphylum Vertebrata consists of about 43,700 species of animals with backbones. Vertebrates exhibit all three of the chordate characteristics at some point during their lives. The embryonic notochord is replaced by a vertebral column in the adult. The vertebral column is made of individual hard segments (vertebrae) surrounding the dorsal hollow nerve cord. The nerve cord is the one chordate feature present in the adult phase of all vertebrates. The vertebral column, part of a flexible but strong endoskeleton, is evidence that vertebrates are segmented. The vertebrate skeleton is living tissue (either cartilage or bone) that grows as the animal grows.
The endoskeleton and muscles form an organ system that permits rapid and efficient movement. The pectoral and pelvic fins of fishes evolved into jointed appendages that allowed vertebrates to move onto land. The skull, the most anterior component of the main axis of the vertebrate endoskeleton, encases the brain. The high degree of cephalization in vertebrates is accompanied by complex sense organs concentrated in the head region. Eyes developed as outgrowths of the brain. Ears were equilibrium devices in aquatic vertebrates that function as sound-wave receivers in land vertebrates. Vertebrates have a complete digestive system and a large coelom. Their circulatory system is closed, with respiratory pigments contained within blood vessels. Gas exchange is efficiently accomplished by gills, lungs, and in a few cases, moist skin. Kidneys are efficient in excretion of nitrogenous waste and regulation of water. Reproduction is usually sexual with separate sexes.
Classification of the Vertebrata
The first vertebrates were fishlike. Fishes are aquatic, gill-breathing vertebrates that usually have fins and skin covered with scales. The larval form of a modern-day lamprey, which looks like a lancelet, may resemble the first vertebrates: it has the three chordate characteristics (like the tunicate larva), as well as a two-chambered heart, a three-part brain, and other internal organs that are like those of vertebrates.
Small, jawless, and finless ostracoderms were the earliest vertebrates. They were filter feeders, but probably were also able to move water through their gills by muscular action. Ostracoderms have been found as fossils from the Cambrian through Devonian periods, when the group finally went extinct. Although extant jawless fishes lack protection, many early jawless fishes had large defensive head shields.
These long, eel-like, jawless fish are free-swimming predators on other fish. Lampreys hatch in freshwater and many live their lives entirely in freshwater. Some lampreys migrate to the sea, but must return to freshwater to reproduce. Lampreys have a sucker-like mouth that lacks a jaw.
Sea lamprey on lake trout
Sea lamprey mouth
Class Myrini, Hagfish
Members of the class Myxini have a partial cranium (skull), but no vertebrae. Their skeleton is made of of cartilage, as is that of sharks. Hagfish lack jaws, and for this reason used to be classified with the lampreys in a group called the Agnatha (“no jaws”) or the Cyclostomata (“round mouth”).
Fish: Vertebrates With Jaws
The fish first appeared during the Cambrian Period. Whether fish first evolved in fresh or salt water is unclear from the fossil record. The jawless fish are the most primitive group, although they were a very important group during the Silurian and Devonian periods. Hagfish and lampreys are the only living members of this class today. They have long, cylindrical bodies with cartilage skeletons and no paired fins.
The first jawed fish were the Placoderms, an extinct group of Devonian-aged jawed fishes. Placoderms were armored with heavy plates and had strong jaws and paired pectoral and pelvic fins. Paired fins allow fish to balance and to maneuver well in water, which facilitate both predation and escape.
The fossil is a cast of the placoderm, Bothriolepis
The evolution of jaws is an example of evolutionary modification of existing structures to perform new functions. Jaws are modified gill arches, and allowed the exploitation of new roles in the habitats: predators with powerful jaws. There are two classes of jawed fish: the cartilaginous fish and the bony fish.
Steps in the evolution of jaws by modification of gill arches.
Class Chondrichthyes: Cartilaginous Fish
The class Chondrichthyes contains approximately 850 species of skates, rays, and sharks. They have jaws, lots of teeth, paired fins, and a cartilage endoskeleton. Cartilaginous fish first appeared during the Devonian Period and expanded in diversity during the Carboniferous and Permian before nearly disappearing during the great extinction that occurred near the end of the Permian. A large group of cartilagenous fish still survives today and is an important part of the marine fauna.
These fish have five to seven gill slits on both sides of the pharynx, and lack the gill covers found in bony fish. The chondrichthyian body is covered epidermal placoid (or toothlike) scales. Developmental studies show the teeth of sharks are enlarged scales.
The largest sharks are filter feeders, not the predators of Hollywood movies. Basking and whale sharks eat tons of crustaceans (small krills, etc.) filtered from the water. Most sharks are fast-swimming, open-sea predators. The great white shark feeds on dolphins, sea lions and seals (and people sometimes). In other words, anything is WANTS to!
Shark and Ray
Rays and skates live on the ocean floor; their pectoral fins are enlarged into winglike fins; they swim slowly. Stingrays have a venomous spine. The electric ray family can feed on fish that have been stunned with electric shock of over 300 volts. Sawfish rays have a large anterior “saw” that they use to slash through schools of fish.
Class Osteichthyes, the Bony Fish
There are about 20,000 species of bony fish, found both in marine and freshwater, comprising the class Osteichthyes. This class is divided into two groups: the lobe-finned (Sarcopterygii) and ray-finned fish (Actinopterygii). The bony fish have a bony skeleton. Most species in this class are ray-finned with thin, bony rays supporting the fins. A few fishes are lobe-finned and are thought to be related to the ancestors of amphibians.
Cross section of a fish
Ray-finned Fish (Actinopterygii)
The ray-finned fish include familiar species such as tuna, bass, perch, and trout. Ray-finned fish are the most successful and diverse of the vertebrates (more than half of all vertebrate species belong to this group). Thin, bony supports with radiating bones (hence the term ray-finned) hold the fins away from the body. Ray-finned fish obtain their food by filter feeding and by preying on insects and other animals. Their skin is covered by scales formed of bone. These scales are homologous to our own hair (and the feathers of birds), being derived from the same embryonic tissues. The gills in this group of fish do not open separately and are covered by an operculum. Ray-finned fish have a swim bladder, a gas-filled sac, that regulates buoyancy and depth. Sharks lack this feature, which enables fish to “sleep” without sinking. The swim bladder acts much the way a ballast tank does on a submarine to control buoyancy.
Salmon, trout, and eels can migrate from fresh water to salt water, but must adjust kidney and gill function to the tonicity of their environments. In freshwater, the fish is hypotonic relative to its aqueous (watery) environment. Water is constantly flooding into the fish, and must be removed by the fish’s excretory system. In seawater, the fish is now hypertonic or isotonic relative to the seawater, requiring conservation of body water.
Bony fishes depend on color vision to detect both rivals and mates. Sperm and eggs are released into the water, with not much parental care for the newborn. Most fish have fertilization and embryonic development taking place outside the female’s body.
Lobe-finned Fish (Sarcopterygii)
This group includes six species of lungfishes and one species of coelacanth that has muscular fins with large, jointed bones attaching the fins to the body. Lobe-finned fish have fleshy fins supported by central bones, homologous to the bones in your arms and legs. These fins underwent modification, becoming the limbs of amphibians and their evolutionary descendants such as lizards, canaries, dinosaurs, and humans.
The lungfish are a small group found mostly in freshwater stagnant water or ponds that dry up in Africa, South America, and Australia.
Coelacanths live in deep oceans. They were once considered extinct, although more than 200 have been captured since 1938. Mitochondrial DNA analysis supports the hypothesis that lungfish are probably the closest living relatives of amphibians.
Coelacanth, a living fossil.
The crossopterygian fish (represented by the marine extant deep-living coelacanth and extinct freshwater forms) are regarded as ancestors of early amphibians. Extinct crossopterygians had strong fins, lungs, and a streamlined body capable of swimming as well as traveling short distances out of water.
Comparison of the skeletons of a crossopterygian lobe-finned fish and an early amphibian.
The term “tetrapod” (meaning four-limbed or four-footed) has historically been applied to the land vertebrates (amphibians, reptiles, dinosaurs, birds, and mammals). All other animals from this point have four limbs and are called tetrapods.
Most zoologists would accept that the Devonian lobe-finned fishes were ancestral to the amphibians. Animals (both vertebrate as well as many invertebrates such as insects) that live on land use limbs to support the body, especially since air is less buoyant than water. Lobe-finned fishes and early amphibians also had lungs and internal nares to respire air.
Two hypotheses have been proposed to explain the evolution of amphibians from lobe-finned fishes.
- Lobe-finned fishes capable of moving from pond-to-pond had an advantage over those that could not.
- The supply of food on land, and the absence of predators, promoted adaptation to land.
The first amphibians diversified during Carboniferous Period (commonly known as the Age of Amphibians).
Class Amphibia: Animals Move Ashore
This class includes 4000 species of animals that spend their larval/juvenile stages in water, and their adult life on land. Amphibians must return to water to mate and lay eggs. Most adults have moist skin that functions in helping their small, inefficient lungs with gas exchange. Frogs, toads, newts, salamanders, and mud puppies are in this transitional group between water and land.
Amphibian features not seen in bony fish include:
- Limbs with girdles of bone that are adapted for walking on land.
- A tongue that can be used for catching prey as well as sensory input.
- Eyelids that help keep the eyes moist.
- Ears adapted for detecting sound waves moving through the thin (as compared to water) medium of the air.
- A larynx adapted for vocalization.
- A larger brain than that of fish, and a more developed cerebral cortex.
- Skin that is thin, smooth, non-scaly, and contains numerous mucous glands; the skin plays an active role in osmotic balance and respiration.
- Development of a lung that is permanently used for gas exchange in the adult form, although some amphibians supplement lung function by exchange of gases across a porous (moist) skin.
- A closed double-loop circulatory system that replaces the single-loop circulatory path of fish.
- Development of a three-chambered heart that pumps mixed blood before and after it has gone to the lungs.
Reproduction involves a return to the water. Ther term “amphibian” refers to two life styles, one in water, the other on land. Amphibians shed eggs into the water where external fertilization occurs, as it does in fish. Generally, amphibian eggs are protected by a coat of jelly but not by a shell. The young hatch into aquatic larvae with gills (tadpoles). Aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult.
Amphibians, like fish, are ectothermic; they depend upon external heat to regulate body temperatures. If the environmental temperature becomes too low, ectotherms become inactive.
Salamanders more likely resemble earliest amphibians due to their S-shaped movements. Salamanders practice internal fertilization; males produce a spermatophore that females pick up. Frogs and toads are tailless as adults, with their hind limbs specialized for jumping.
Class Reptilia: Reproducton Without Water
This class of 6000 species includes the snakes, lizards, turtles, alligators, and crocodiles. Reptiles that lay eggs lay an egg surrounded by a thick protective shell and a series of internal membranes. Reptiles have internal fertilization: their gametes do not need to be released into water for fertilization to occur.
The amniotic egg is a superb adaptation to life on land. While amphibians need to lay their eggs in water, their descendants (reptiles) were not as strongly tied to moist environments and could truly expand into more arid areas. Reptiles were the first land vertebrates to practice internal fertilization through copulation and to lay eggs that are protected by a leathery shell with food and other support for the growing embryo.
The amniote egg contains extra-embryonic membranes that are not part of the embryo and are disposed of after the embryo has developed and hatched. These membranes protect the embryo, remove nitrogenous wastes, and provide the embryo with oxygen, food, and water. The amnion, one of these extra-embryonic membranes, creates a sac that fills with fluid and provides a watery environment in which the embryo develops. The embryo develops in a “pond within the shell”.
Structure of the amniote egg
Evolutionary History of Reptiles
Reptiles first evolved during the Carboniferous time and partly displaced amphibians in many environments. The first reptiles (often referred to as the stem reptiles) gave rise to several other lineages, each of which adapted to a different way of life. Reptilian success was due to their terrestrial (amniotic) egg and internal fertilization, as well as their tough leathery skin, more efficient teeth and jaws, and in some, bipedalism (traveling on their hind legs, allowing the forelimbs to grasp prey or food, or become wings). One group, the Pelycosaurs (fin-backed or sail lizards) are related to therapsids, mammal-like reptiles ancestral to mammals. Other groups returned to aquatic environments. Ichthyosaurs were fishlike (or dolphin-like) free-swimming predators of the Mesozoic seas. The plesiosaurs had a long neck and a body adapted tp swimming though use of flippers (legs that evolutionarily reverted to a flipper-like shape). These free-swimmers also adapted to live birth of their young (since they could not return to the land to lay eggs). Thecodonts were the reptiles that gave rise to most of the reptiles, living and extinct. Pterosaurs were flying reptiles that dominated the Mesozoic skies. They had a keel for attachment of flight muscles and air spaces in bones to reduce weight.
Dinosaurs (descended from some thecodonts) and mammal-like reptiles’ had their limbs beneath the body providing increased agility and facilitating gigantic size. Lizards have their elbows out (like you do when you do a push-up). By having their elbows in, dinosaurs and mammals place more of the weight of the body on the long bones instead of the elbows, ankles, and knees.
Relationship between limbs and body. Note that reptiles have their upper limbs jutting out from the body, while mammals have their limbs in line with the body, supporting and more easily raising the body mass off the ground.
Reptiles dominated the earth for about 170 million years during the Mesozoic Era. The mass extinction of many reptile groups at the close of the Mesozoic (the Cretaceous Period) has been well documented and the subject of many hypotheses. The 1980 hypothesis by Luis and Walter Alvarez and others proposes the impact of a large meteorite at the end of the Cretaceous period caused a catastrophic environmental collapse that led to the extinction of nearly 50% of all species of life on Earth. The survivors, birds and mammals, reaped the spoils and diversified during the Cenozoic Era. Three groups of reptiles remain: turtles, snakes/lizards, and crocodiles/alligators.
About 6,000 species of reptiles comprise the Class Reptilia. Most live in tropics or subtropics. Lizards and snakes live on land, while turtles and alligators live in water for much of their lives. Reptiles have a thick, scaly skin that is keratinized and impermeable to water. This same keratin is a protein found in hair, fingernails, and feathers. Protective skin prevents water loss but requires several molts a year. Reptilian lungs are more developed than those of amphibians. Air moves in and out of the lungs due to the presence of an expandable rib cage in all reptiles except turtles. Most reptiles have a nearly four-chambered heart. The crocodile has a completely four-chambered heart that more fully separates oxygen-rich blood from from deoxygenated or oxygen-poor blood. The well-developed kidneys excrete uric acid; less water is lost in excretion. Reptiles are ectothermic; they require a fraction of the food per body weight of birds and mammals, but are behaviorally adapted to warm their body temperature by sunbathing.
Photograph of a lizard (L) and a gavial (R)
Snakes and lizards live mainly in the tropics and desert. Lizards have four clawed legs and are carnivorous; marine iguanas on the Galapagos are adapted to spend time in the sea; frilled lizards have a collar to scare predators, and blind worm lizards live underground. Snakes evolved from lizards and lost their legs as an adaptation to burrowing. Their jaws can readily dislocate to engulf large food. The snake’s tongue collects airborne molecules and transfers them to the Jacobson’s organ for tasting. Some poisonous snakes have special fangs for injecting their venom.
Turtles have a heavy shell fused to the ribs and thoracic vertebrae; they lack teeth but use a sharp beak; sea turtles must leave the ocean to lay eggs onshore.
Crocodiles and alligators are largely aquatic, feeding on fishes and other animals. They both have a muscular tail that acts as a paddle to swim and a weapon. The male crocodile bellows to attract mates. In some species the male also protects the eggs and young.
The Archosauria: Birds, Dinosaurs, and More
Cladistic analyses place the birds, alligators, and dinosaurs in the same clade, the Archosauria (or “ruling reptiles”). This group is a major group of diapsids (vertebrates that have two openings in their skulls) that have single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics. This helps to lighten the skull, provides more room for muscles and other tissues, and allows more skull flexibility when eating. Other typical archosaurian characteristics include another opening in the lower jaw (the mandibular fenestra), a high narrow skull with a pointed snout, teeth set in sockets, and a modified ankle joint.
The ancestral archosaurs probably originated some 250 million years or so ago, during the late Permian period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a most of the Mesozoic Era. The birds and crocodilians are the last living groups of archosaurs.
Class Aves: Birds of a Feather
The class Aves (birds) contains about 9000 species. Birds evolved from either a dinosaurian or other reptilian group during the Jurassic (or possibly earlier). The earliest bird fossils, such as the Jurassic Archaeopteryx or Triassic Protavis, display a mosaic of reptilian and bird features (teeth in the bill, a jointed tail, and claws on the wing are reptilian; feathers and hollow bones are bird-like).
Archaeopteryx, once considered the first bird.The fossil is from the Solenhoefen Limestone (Jurassic) of Germany
The distinguishing feature of birds is feathers: which provide insulation as well as aid in flight.
Structure of a feather
Remember, not all animals that fly have feathers, but all almost every endothermic animal (warm-blooded) has a covering of hair or feathers for insulation. The recent (1999) discovery of a “feathered” dinosaur adds credence to this speculation. The dinosaur could not fly, so of what use would feather be but insulation (or possibly mating).
Modern birds appeared during the early Tertiary, and have adapted to all modes of life: flying (condors, eagles, hummingbirds), flightless-running (ostriches, emus), and swimming (penguins). Birds exhibit complex mating rituals as well as social structure (a pecking order!).
Class Mammalia: Got Milk?
Class Mammalia contains around 5000 species placed in 26 orders (usually). The three unifying mammalian characteristics are:
- the presence of three middle ear bones
- the production of milk by mammary glands
Milk is a substance rich in fats and proteins. Mammary glands usually occur on the ventral surface of females in rows (when there are more than two glands). Humans and apes have two mammary glands (one right, one left), while other animals can have a dozen or more. All mammals have hair at some point during their life. Mammalian hair is composed of the protein keratin. Hair has several functions: 1) insulation; 2) sensory function (whiskers of a cat); 3) camouflage, a warning system to predators, communication of social information, gender, or threats; and 4) protection as an additional layer or by forming dangerous spines that deter predators. Modifications of the malleus and incus (bones from the jaw in reptiles) work with the stapes to allow mammals to hear sounds after they are transmitted from the outside world to their inner ears by a chain of these three bones.
Mammals first evolved from the mammal-like reptiles during the Triassic period, about the same time as the first dinosaurs. However, mammals were minor players in the world of the Mesozoic, and only diversified and became prominent after the extinction of dinosaurs at the close of the Cretaceous period.
Mammals have since occupied all roles once held by dinosaurs and their relatives (flying: bats; swimming: whales, dolphins; large predators: tigers, lions; large herbivores: elephants, rhinos), as well as a new one (thinkers and tool makers: humans). There are 4500 species of living mammals.
- Mammals developed several adaptations that help explain their success.
- Teeth are specialized for cutting, shearing or grinding; thick enamel helps prevent teeth from wearing out.
- Mammals are capable of rapid locomotion.
- Brain sizes are larger per pound of body weight than most other animals’.
- Mammals have more efficient control over their body temperatures than do birds.
- Hair provides insulation.
- Mammary glands provide milk to nourish the young.
Subclass Prototheria: Order Monotremata: Monotremes (typified by the platypus and echinda) lay eggs that have similar membranes and structure to reptilian eggs. Females burrow in ground and incubates their eggs. Both males and females produce milk to nourish the young There are two families living today and quite a few known from the fossil record of Gondwana. Monotremes are today restricted to Australia and New Guinea. The earliest fossil monotreme is from the early Cretaceous, and younger fossils hint at a formerly more widespread distribution for the group. While their fossil record is scarce, zoologists believe that monotremes probably diverged from other mammals during the Mesozoic. Monotremes have many differences with other mammals and are often placed in a separate group, the subclass Prototheria. They retain many characters of their therapsid ancestors, such as laying eggs, limbs oriented with humerus and femur held lateral to body (more lizard-like), a cloaca, skulls with an almost birdlike appearance, and a lack of teeth in adults. This suggests that monotremes are the sister group to all other mammals. However, monotremes do have all of the mammalian defining features of the group.
Subclass Metatheria: Marsupials (such as the koala, opossum, and kangaroo) are born while in an embryonic stage and finish development outside the mother’s body, often in a pouch. Marsupial young leave the uterus, crawl to the pouch, and attach to the nipple of a mammary gland and continue their development. Marsupials were once widespread, but today are dominant only in Australia, where they underwent adaptive radiation in the absence of placental mammals. The Metatheria contains 272 species classified in several orders. Metatheres diverged from the lineage leading to the eutherian (placental) mammals by the middle of the Cretaceous period in North America. The earliest marsupial fossils resemble North American opossums. Marsupial fossils are found on other northern hemisphere continents, although they seem not to have been prominent elements of those faunas. On the other hand, in South America and Australia, marsupials continued to be dominant faunal elements. The marsupials of South America began to go extinct in the late Miocene and Early Pliocene (Cenozoic era) when volcanic islands grew together and formed the Isthmus of Panama, allowing North American placental mammals to cross into South America. Australian marsupials remain diverse and dominant native mammals of the fauna. During the Cenozoic Era many marsupials in South America and Australia underwent parallel (or convergent) evolution with placental mammals elsewhere, producing marsupial “wolves”, “lions”, and saber-toothed marsupial “cats”.
Red Kangaroo with its joey
Subclass Eutheria: There are 4000 described species of placental mammals, a group that includes dogs, cats, and people. The subclass is defined by a true placenta that nourishes and protects the embryos held within the mother’s body for an extended gestation period (nearly two years for an elephant, and nine very long months for a human). The eutherian placenta has extraembryonic membranes modified for internal development within the uterus. The chorion is the fetal portion of placenta, while the uterine wall grows the maternal portion. The placenta exchanges nutrients, oxygen, and wastes between fetal and maternal blood.
There are 12 orders of placental mammals. Classification is based on the mode of locomotion and methods of obtaining food. Prominent orders include the bats (order Chiroptera), horses (order Perissodactyla), whales (order Cetacea), mice (order Rodentia), dogs (order Carnivora), and monkeys/apes/humans (order Primates).
1. In what phylum & kingdom are the vertebrates found?
2. List the classes of vertebrates.
3. Discuss the characteristics of chordates & vertebrates.
4. What were the 1st vertebrates & describe them?
5. Sketch a lamprey & describe the characteristics of this fish. Where are they found?
6. Describe a hagfish.
7. In what group are lampreys & hagfish found & why?
8. Do agnathans have paired fins?
9. What were the 1st jawed fish & describe them.
10. What are the 2 classes of jawed fish?
11. What is in the class Chondrichthyes & what traits do they have in common.
12. Sketch & describe sharks.
13. Sketch a ray or skate & describe them.
14. Name the class for bony fish.
15. Name the 2 groups of bony fish.
16. Give several examples of ray-finned fish & describe them.
17. Name 2 lobe-finned fish & describe both of them.
18. What was the 1st group of vertebrates to move onto land? What is in this group?
19. Describe characteristics of amphibians.
20. Amphibians are ectotherms. What does this mean?
21. How are amphibians still linked to water?
22. What is in the class Reptilia?
23. Reptiles do not need water for reproduction. Explain why this is true.
24. Describe the amniote egg of reptiles. Include a labeled sketch of the egg.
25. What reptile group is thought to be the ancestors of mammals?
26. What were pterosaurs?
27. What 3 groups of retiles are still alive today?
28. Describe characteristics of the reptiles.
29. How can snakes swallow such large prey?
30. What is the purpose of the Jacobson’s organ in snakes?
31. What takes the place of teeth in turtles?
32. Describe crocodiles & alligators & tell some of their habits.
33. What class contains birds?
34. From what did birds probably evolve?
35. What are the distinguishing features of birds?
36. Sketch & label the parts of a feature.
37. Birds are endotherms. What does this mean?
38. Name some flightless birds.
39. Name some swimming birds.
40. What are the 3 main characteristics of all mammals?
41. What in female mammals produces milk?
42. What is mammalian hair made of & give its 4 functions.
43. What bones are modified in mammals to help them hear sounds?
44. Name a flying mammal.
45. Give examples of mammals that are herbivores.
46. Give examples of mammals that are carnivores.
47. What mammal is a thinker & toolmaker?
48.Name 7 adaptations of mammals.
49.Give examples of monotremes & tell their characteristics. Tell where they are found.
50. Give examples of marsupials & tell their characteristics. Tell where most of them are found.
51. Most mammals are placentals. What does this mean?
52. What is gestation? Do all mammals have the same gestation period?
53. What is the purpose of the chorion?
54.Name the 12 orders of placental mammals & give an example of an animal in each order.
Mammal Orders Solution
|Reptiles All Materials © Cmassengale|
Evolution of Reptiles:
- Reptiles were 1st vertebrates to make a complete transition to life on land (more food & space)
- Arose from ancestral reptile group called cotylosaurs (small, lizard like reptile)
- Cotylosaurs adapted to other environments in Permian period
1. Pterosaurs – flying reptiles
2. Ichthyosaurs & plesiosaurs – marine reptiles
3. Thecodonts – small, land reptiles that walked on back legs
- Mesozoic Era called “age of reptiles”
- Dinosaurs dominated life on land for 160 million years
- Brachiosaurs were largest dinosaurs
- Herbivores included Brontosaurus & Diplodocus, while Tyrannosaurus were carnivores
- Dinosaurs became extinct at end of Cretaceous period
- Mass extinction of many animal species possibly due to impact of huge asteroid with earth; Asteroid Impact Theory
- Amniote (shelled) egg allowed reptiles to live & reproduce on land
- Egg had protective membranes & porous shell enclosing the embryo
- Has 4 specialized membranes — amnion, yolk sac, allantois, & chorion
- Amnion is a thin membrane surrounding a salty fluid in which the embryo “floats”
- Yolk sac encloses the yolk or protein-rich food supply for embryo
- Allantois stores nitrogenous wastes made by embryo until egg hatches
- Chorion lines the inside of the shell & regulates oxygen & carbon dioxide exchange
- Shell leathery & waterproof
- Internal fertilization occurs in female before shell is formed
- Dry, watertight skin covered by scales made of a protein called keratin to prevent desiccation (water loss)
- Toes with claws to dig & climb
- Geckos have toes modified into suction cups to aid climbing
- Snakes use scales & well developed muscular & skeletal systems to move
- Lungs for respiration
- Double circulation of blood through heart to increase oxygen to cells
- Partial separation in ventricle to separate oxygenated & deoxygenated blood
- Ectothermic – body temperature controlled by environment
- May bask or lie in sun to raise body temperature or seek shade to lower body temperature; known as thermoregulation
- Water conserved as nitrogen wastes excreted in dry, paste like form of uric acid crystals
- Only 4 living orders remain
- Found worldwide except in coldest ecosystems
- Orders include —– Rhyncocephalia (tuatara lizard), Chelonia (turtles & tortoises), Squamata (lizards & snakes), & Crocodilia (alligators, caimans, and crocodiles)
- Only one living species, Spenodon punctatus, (tuatara lizard)
- Live on islands off the coast of New Zealand
- Spiny crest running down back
- Grows up to 60 cm in length
- Has 3rd eye on top of head (parietal eye) that acts as a thermostat
- Most active when temperatures are low (nocturnal)
- Often burrow during the day
- Feed on insects, worms, & small animals at night
- Includes turtles and tortoises
- Aquatic, but lay eggs on land
- Body covered with shell composed of hard plates & tough, leathery skin
- Carapace or dorsal surface of shell fused with vertebrae & ribs
- Plastron is ventral shell surface
- Shape of shell modified for habitat
- Dome shaped shell helps to retract head & limbs in tortoises
- Water-dwelling turtles have streamline, disk shaped shell to rapidly move in water
- Forelimbs of marine turtles modified into flippers
- River & sea turtles migrate to breeding areas where they hatched to lay their eggs on land
- Includes crocodiles, alligators, caimans, & gavials
- Direct descendants of Archosaurs
- Carnivorous (wait for prey to come near & then aggressively attack)
- Eyes located on top of head so they can see when submerged
- Nostrils on top of snout to breathe in water
- Valve in back of mouth prevents water from entering airway when feeding underwater
- No parental care of young in most species except Nile crocodile that carry young in their jaws & guards nest
- Crocodiles are tropical or subtropical, usually nocturnal, reptiles found in Africa, Asia, South America, & southern Florida
- Alligators are found in China & the southern United States
- Caimans are native to Central America & resemble alligators
- Gavials, living only in India & Burma, are fish eating reptiles with very slender, long snouts
- Includes snakes & lizards
- Snakes probably evolved from lizards during the Cretaceous period
- Snakes have 100-400 vertebrae each with a pair of ribs & attached muscles for movement
- Interaction of bone, muscles, & skin of snakes allows them 3 ways to move — lateral, rectilinear, & side winding
- Lateral undulations:
1. Most common
2. Head moves side to side causing wave of muscular contractions
3. Snake uses sides of its body to push off of ground
4. Snake moves forward in S-shaped path
- Rectilinear Movements:
1. Muscular force applied to belly & not sides of snake
2. Scutes or scales on belly catch on rough surfaces
3. Body relaxes & then moves forward slowly
1. Used by some desert snakes
2. Sideways movement of body
3. Head vigorously flung from side to side
4. Whiplike motion moves body along
- Do not hear or see well but locate prey using forked tongue that gathers chemical scents
- Swallow prey whole:
1. Jaws unhinge for mouth to stretch
2. Small teeth used to hold prey in mouth
3. Windpipe thrust into throat while swallowing so snake can swallow & breathe
4. Swallowing may take several hours
5. Saliva begins digestion during swallowing
- Constrictors wrap body around prey & squeeze them to death (boas, pythons, etc.)
- Snakes may inject venom or poison:
1. Hemotoxin – poisonous proteins attacking red blood cells (water moccasin & rattlesnake)
2. Neurotoxin – poison that works on nervous system affecting heart rate & breathing (copperhead)
- Venomous snakes with 3 types of fangs — rear-fanged, front-fanged, & hinge- fanged snakes
- Rear-fanged snakes bite prey & use grooved back teeth to guide venom into puncture (boomslang)
- Front-fanged snakes inject poison through 2 small front fangs that act like a hypodermic needle (cobra)
- Hinged- fang snakes have hinged fangs in roof of mouth that swing forward to inject poison (rattlesnake, water moccasin, copperhead)
- Often camouflaged for defense
- May use signals such as cobra expanding its hood, rattlesnake shaking its rattle, or hissing for defense
- Most snakes locate females by scent
- Internal fertilization with no parental care
- May be oviparous (eggs hatch outside body) or ovoviviparous (eggs held inside body until hatch)
1. Four limbs
2. Includes iguanas, geckos, skinks, chameleons, etc.
3. Rely on speed, agility, & camouflage to catch prey
4. Feed on insects & small worms
5. Some, such as anole & chameleon, can change colors for protection
6. May use active displays such as squirting blood, hissing, or inflating bodies
7. Some show autotomy (breaking off tail to escape predators)
8. Two poisonous U.S. species include Gila Monster & Beaded Lizard
- Komodo dragon of Indonesia is largest lizard reaching 3 meters in length
All Materials © Cmassengale
Main Characteristics of mammals:
- Endothermy – maintain high, constant body temperature through their metabolism
- Pelage – hair or fur made of protein called keratin covering all or part of the body for insulation & camouflage
- Four chambered heart ( two atria & two ventricles) keep oxygenated & deoxygenated blood from mixing; double circulation
- Mammary glands in females are modified sweat glands that make milk containing sugars, proteins, & fats to nourish young
- Single jawbone
- Specialized teeth for biting, cutting, & chewing
- Highly developed brain (large cerebrum)
- Diaphragm – muscle below lungs that aids respiration
- Most are viviparous (live birth)
- Uterus in females where young develop
- Placenta lines uterus & provides nutrients and gas & waste exchange for developing young
- Have sweat glands for cooling & scent glands for attracting mates & marking territories
- Fossil records show mammals arose from group of reptiles called therapsids at the end of the Paleozoic era
- Therapsids were endotherms with specialized teeth like mammals
- First mammalian fossil found in Mesozoic era (hair, single jawbone, specialized teeth, & endothermic)
- Early mammals were small, shrew like, insect eaters that had large eye sockets making them probably nocturnal
- When dinosaurs became extinct, new habitats & food supplies opened up for mammals
- “Age of mammals” occurred during Cenozoic era
- Oviparous (egg laying) monotremes evolved first
- Viviparous (live birth) marsupials with incomplete uterine development appeared next & then placental mammals
Specializations of the mouth & digestive system:
- Single jawbone
- Incisors – specialized, chisel like front teeth for biting & chewing
- Canines – pointed teeth or fangs behind incisors to help grip, puncture, & tear prey
- Bicuspids – teeth with two points behind the canines used to shear & shred food
- Molars – flattened back teeth to grind & crush
- Baleen – thin plates in the roof of the mouth of some whales that strain food from water
- Microorganisms living in the gut help some mammals digest cellulose from plants
- Hoofed mammals (cows, sheep, giraffes…) have a four-chambered stomach with bacteria living in the first chamber or rumen
- Cud – digested food in the rumen that is regurgitated, swallowed, & then chewed again to break down plant cellulose
- Caecum – stomach chamber in elephants, horses, & rabbits that contains bacteria to digest cellulose
Adaptations for Endothermy:
- High demand for oxygen
- Right & left sides of heart separated by septum so oxygenated & deoxygenated blood don’t mix
- Left side of heart pumps blood to lungs & back (pulmonary circulation)
- Right side of blood pumps oxygenated blood to body cells (systemic circulation)
- Diaphragm – sheet of muscle below lungs that moves up & down in chest to change air pressure so gas moves into & out of the lungs
- Alveoli or air sacs in the lungs are surrounded by capillaries and increase the surface area for the absorption of oxygen
- Hair or fur and a fat layer insulates and prevents heat loss
Nervous System Adaptations:
- Largest vertebrate brain
- Cerebrum surface is folded to increase surface area without increasing volume
- Cerebrum controls sensory organs, coordinates movement, regulates behavior, & is responsible for memory & learning
- Have five major senses — vision, hearing, olfaction (smell), touch, & taste
- Bats, whales, dolphins, porpoises use echolocation (bouncing off of high frequency sounds) to navigate & find prey
- Each of the 3 mammal groups — monotremes, marsupials, & placentals— has a unique reproductive pattern
- Monotreme females lay 1-2 leathery-shelled eggs containing yolk & incubates them with her body heat
- Young monotremes are small & partially developed at hatching so depend on mother for protection and milk from mammary glands
- Marsupials have short development period inside of the mother & newborns must crawl to the mother’s pouch or marsupium after birth, attach to a nipple for milk, and finish developing
Mother Kangaroo & “Joey”
- Placentals are the largest group of mammals
- Gestation (period of development inside mother) is longer in placental mammals
- Nutrients, wastes, gases exchanged through membrane lining uterus called the placenta
|Section 2 Review|
- Not completely endothermic (lower body temperature & it fluctuates)
- Have a cloaca where wastes, eggs, & sperm are emptied
- Includes duck-billed platypus & spiny anteaters or echidna
- Live only in Australia & New Zealand
1. Waterproof fur
3. Flattened tail for swimming
4. Flat, sensitive, rubbery muzzle used to root for worms & crayfish
5. Digs a den in bank of river to lay eggs
6. Female curls around eggs & incubates them
7. Newborns lick milk from nippleless mammary glands
2. Coat of protective spines
3. Long snout to probe ant hills & termite nests
4. Incubate eggs in a brood pouch on female’s belly
- Found in New Guinea, Australia, & the Americas
- Dominate animal in Australia due to lack of competition from placental mammals
- Known as pouched animals
- Pouch called marsupium
- Viviparous (live birth)
- Tiny, immature young must crawl to mother’s pouch after birth
- Young attach to mammary gland nipple to nurse until able to survive outside of pouch
- Includes opossum, kangaroo, wombat, & koala
|Koala & baby||Opossum|
Placental Mammals :
- Young carried in uterus & nourished by placenta
- Gestation periods (time of development within uterus) varies among species
- Adapted for life on land in water, and in air
- Mammal species make up 95 % of all animals
- At least 18 orders exist
- Includes moles, hedgehogs, & shrews
- Small with high metabolic rate
- Found in North America, Europe, & Asia
- Have long, pointed noses to grub for insects & worms
- Teeth adapted to pick up & pierce prey
- Adapted to live on & under ground, in trees, and in water
- Shrews feed above ground & have claws to help sweep invertebrates into their mouths
- Moles live underground, have reduced eyes & no external ears, and have short limbs to dig tunnels
- Largest mammal order (40% of all species)
- Found everywhere except Antarctica
- Includes squirrels, chipmunks, gophers, rats, mice, & porcupines
- Have two instead of four incisors
- Teeth continue to grow throughout their life
- Feed on hard seeds, twigs, roots, & bark
- Gnawing keeps incisors sharp
- High reproductive capacity
- Guinea pig & capybaras are two rodents found in South America
- includes rabbits, hares, & pikas
- Found worldwide
- Have a double row of upper incisors & two large front teeth backed up by two smaller teeth
- Continuous growing teeth
- Includes anteaters, armadillos, & sloths
- Found in North, Central, & South America
- Means “without teeth”
- Only anteaters are completely toothless
- Armadillos & sloths have peg-like teeth without enamel
- Have long sticky tongues & claws on powerful front paws to open ant hills& termite nests
- Sloths are herbivores
- Armadillos eat small reptiles, frogs, mollusks, & dead animals
- Only flying mammals
- Includes bats found everywhere except polar regions
- Front limb is modified into a wing with a skin membrane stretching from the finger bones to the hind limb
- Clawed thumb, extending from top edge of wing, is used for walking, climbing, & grasping
- Most are nocturnal (night active)
- Use echolocation (emission of high frequency sounds that bounce off objects) to navigate and locate food
- Have small eyes & large ears
- Feed mainly on insects
- Tropical bats don’t use echolocation, but have large eyes & keen sense of smell to find fruit to feed on & nectar
- Includes whales, dolphins, & porpoises
- Most inhabit oceans, but some dolphins live in freshwater rivers
- Have a fish shaped body
- Forelimbs modified as flippers
- No hind limbs
- Broad, flat tails to propel through water
- Breathe through a blowhole on top of the head
- Divided into two groups — toothed whales & baleen whales
- Toothed whales:
1. Includes beaked, sperm, beluga, & killer whales; narwhals; dolphins; porpoises
2. Have 1 to more than 100 teeth
3. Prey on fish, squid, seals, & other whales
- Baleen whales:
1. Lack teeth
2. Includes blue, grey, right & humpbacked whales
2. Have baleen or thin plates of fingernail like material that hangs from the roof of the mouth
3. Baleen strain shrimp & other invertebrates from water as food
|Blue Whale||Humpbacked Whale|
- Includes manatees & dugongs
- Large herbivores
- Inhabit tropical seas, estuaries, & rivers
- Front limbs modified into flippers
- No hind limbs
- Flattened tail for propulsion
- Found worldwide
- Includes cats, dogs, raccoons, bears, hyenas, & otters
- Meat eaters (carnivores) mainly
- Many feed on both plants & animals (omnivores)
- Have long canine teeth & strong jaws
- Clawed toes for seizing & holding prey
- Keen sense of sight & smell
- Long limbs for running fast
- Aquatic carnivores
- Includes sea lions, seals, & walruses
- Streamlined bodies adapted for swimming
- Steer & propel through water using broad, flattened tail
- Called pinnipeds
- Return to land to feed & give birth
- Spend much of their time in cold water
- Large land carnivores so this helps maintain endothermy
- Can remain under water for 5 minutes to an hour for some species
- Known as ungulates or hoofed mammals
- Have an even number of toes
- Includes deer, elk, bison, moose, sheep, cows, caribou, goats, pigs, & camels
- Have large flat molars for grinding plants
- Found everywhere except Antarctica
- Cloven or split hooves
- Fast runners (used for defense)
- Have storage chamber called rumen in stomach where bacteria break down cellulose
- Stored food called cud is chewed again & then swallowed to go through digestive system a second time
- Odd toed ungulates
- Includes horses, zebras, rhinoceroses, & tapir
- Most are native to Africa & Asia
- Tapirs are found in Central & South America
- Have a large, convoluted caecum or blind sac near the small intestine where bacteria digest cellulose
|Caribou (even-toed)||Tapir (odd-toed)|
- Have a boneless trunk or proboscis
- Includes the African & Asian elephant
- Wooly mammoth is an extinct member of this order
- Largest terrestrial mammal
- Weigh more than 6 tons
- Feed on plants up to 18 hours a day
- Proboscis used to gather leaves from high branches & to suck water without lowering the head
- Modified incisors called tusks help dig for roots & strip bark
- Jagged molars up to 30 cm long grind plants
- Have the longest gestation period (20 months for females & 22 months for males)
- Females can continue to have calves until they are 70 years old
|African Elephant||Asian Elephant|
- Includes 2 main groups — Prosimians & Anthropoids
- Most are omnivores
- Have teeth suitable for a varied diet
- Prosimians include lemurs, tarsiers, & lorises
- Anthropoids include monkeys, apes, & humans
- Anthropoids have a larger brain
- Show more complex behaviors than other animals
- Highly organized social groups
- Gorilla is the largest primate
- Have 2 forward-facing eyes for depth perception
- Have grasping hands & most with grasping feet
- Some have a grasping tail for life in trees
- Live in a variety of habitats
|Section 3 Review|
Locate the orders of mammals and then list the common names of the animals in each order
Mammals have four-chambered hearts and double circulation. The heart of a bird or mammal has two atria and two completely separated ventricles. The double-loop circulation is similar to amphibians and reptiles, but the oxygen-rich blood is completely separated from oxygen-poor blood. The left side of the heart handles only oxygenated blood, and the right side receives and pumps only deoxygenated blood. With no mixing of the two kinds of blood, and with a double circulation that restores pressure after blood has passed through the lung capillaries, delivery of oxygen to all parts of the body for cellular respiration is enhanced. As endotherms, which use heat released from metabolism to warm the body, mammals require more oxygen per gram of body weight than other vertebrates of equal size. Birds and mammals descended from different reptilian ancestors, and their four-chambered hearts evolved independently – an example of convergent evolution.
Using a pig heart, students will observe the major chambers, valves, and vessels of the heart and be able to describe the circulation of blood through the heart to the lungs and back and out to the rest of the body. (The pig heart is used because it is very similar to the human heart in structure, size, & function.)
Dissecting pan, dissecting kit, safety glasses, lab apron, pig heart, & gloves
Procedure – External Structure
- Place a heart in a dissecting pan & rinse off the excess preservative with tap water. Pat the heart dry.
- Examine the heart and locate the thin membrane or pericardium that still covers the heart. The pericardium or pericardial sac, is a double-layered closed sac that surrounds the heart and anchors it. The pericardium consists of two tissues layers – the visceral pericardium that covers the surface of the heart & the parietal pericardium covering the inner surface of the parietal sac. These two tissue layers are continuous with each other where the vessels enter or leave the heart. The slender gap between the parietal & visceral surfaces is the pericardial cavity & is filled with fluid to reduce friction between the layers as the heart pumps.
- After examining the pericardium, carefully remove this tissue. Located below the pericardium is the muscle of your heart called the myocardium. Most of the myocardium is located in the lower two chambers of the heart called ventricles.
- Locate the tip of the heart or the apex. Only the left ventricle extends all the way to the apex.
- Place the heart in the dissecting pan so that the front or ventral side is towards you ( the major blood vessels are on the top and the apex is down). The front of the heart is recognized by a groove that extends from the right side of the broad end of the heart diagonally to a point above & to your left of the apex.
Front or Ventral Side of the Heart
- The heart is now in the pan in the position it would be in a body as you face the body. Locate the following chambers of the heart from this surface:
- Left atria – upper chamber to your right
- Left ventricle – lower chamber to your right
- Right atria – upper chamber to your left
- Right ventricle – lower chamber to your left
- While the heart is still in this position in the dissecting pan, locate these blood vessels at the broad end of the heart:
- Coronary artery – this blood vessel lies in the groove on the front of the heart & it branches over the front & the back side of the heart to supply fresh blood with oxygen & nutrients to the heart muscle itself.
- Pulmonary artery – this blood vessel branches & carries blood to the lungs to receive oxygen & can be found curving out of the right ventricle (upper chamber to your left)
- Aorta – major vessel located near the right atria & just behind the pulmonary arteries to the lungs. Locate the curved part of this vessel known as the aortic arch. Branching from the aortic arch is a large artery that supplies blood to the upper body.
- Pulmonary veins – these vessels return oxygenated blood from the right & left lungs to the left atrium (upper chamber on your right)
- Inferior & Superior Vena Cava – these two blood vessels are located on your left of the heart and connect to the right atrium (upper chamber on your left). Deoxygenated blood enters the body through these vessels into the right receiving chamber. Use your probe to feel down into the right atrium. These vessels do not contain valves to control blood flow.
Procedure – Internal Anatomy:
- Use scissors to cut through the side of the pulmonary artery and continue cutting down into the wall of the right ventricle. Be careful to just cut deep enough to go through the wall of the heart chamber. (Your cutting line should be above & parallel to the groove of the coronary artery.)
- With your fingers, push open the heart at the cut to examine the internal structure. If there is dried blood inside the chambers, rinse out the heart.
- Locate the right atrium. Notice the thinner muscular wall of this receiving chamber.
- Find where the inferior & superior vena cava enter this chamber & notice the lack of valves.
- Locate the valve that between the right atrium and right ventricle. This is called the tricuspid valve. The valve consists of three leaflets & has long fibers of connective tissue called chordae tendinae that attach it to papillary muscles of the heart. This valve allows blood flow from the right atrium into the right ventricle during diastole (period when the heart is relaxed). When the heart begins to contract (systole phase), ventricular pressure increases until it is greater than the pressure in the atrium causing the tricuspid to snap closed.
- Use your fingers to feel the thickness of the right ventricle and its smooth lining. Also note the network of irregular muscular cords on the inner wall of this chamber.
- Find the septum on the right side of the right ventricle. This thick muscular wall separates the right & left pumping ventricles from each other.
- Inside the right ventricle, locate the pulmonary artery that carries blood away from this chamber. Find the one-way valve called the pulmonary valve that controls blood flow away from the right ventricle at the entrance to this blood vessel.
- Using your scissors, continue to cut open the heart. Start a cut on the outside of the left atrium downward into the left ventricle cutting toward the apex to the septum at the center groove. Push open the heart at this cut with your fingers & rinse out any dried blood with water.
- Examine the left atrium. Find the openings of the pulmonary veins form the lungs. Observe the one-way, semi-lunar valves at the entrance to these veins.
- Inside this chamber, look for the valve that controls blood flow between the upper left atrium and lower left ventricle. This valve is called the bicuspid or mitral valve. This valve consists of two leaflets & blood flows from the left atrium into the left ventricle during diastole.
- Examine the left ventricle. Notice the thickness of the ventricular wall. This heart chamber is responsible for pumping blood throughout the body.
- Using your scissors, cut across the left ventricle toward the aorta & continue cutting to expose the valve.
- Count the three flaps or leaflets on this valve leading from the left ventricle into the aorta and note their half-moon shape. This is called the aortic valve.
- Using scissors, cut through the aorta and examine the inside. Find the hole or coronary sinus in the wall of this major artery. This leads into the coronary artery which carries blood to and nourishes the heart muscle itself.
- Answer the questions on your lab report.
When you have finished dissecting the heart, dispose of the heart as your teacher advises and clean, dry, and return all dissecting equipment to the lab cart. Wash your hands thoroughly with soap.
Fetal Pig Dissection
Fetal Pig Dissection Background:
Mammals are vertebrates having hair on their body and mammary glands to nourish their young. The majority are placental mammals in which the developing young, or fetus, grows inside the female’s uterus while attached to a membrane called the placenta. The placenta is the source of food and oxygen for the fetus, and it also serves to get rid of fetal wastes. The dissection of the fetal pig in the laboratory is important because pigs and humans have the same level of metabolism and have similar organs and systems. Also, fetal pigs are a byproduct of the pork food industry so they aren’t raised for dissection purposes, and they are relatively inexpensive.
Objectives of fetal pig dissection:
- Identify important external structures of the fetal pig anatomy.
- Identify major structures associated with a fetal pig’s digestive, respiratory, circulatory, urogenital, & nervous systems.
- Compare the functions of certain organs in a fetal mammal with those of an adult mammal.
preserved fetal pig, dissecting pan, dissecting kit, dissecting pins, string, plastic bag, metric ruler, paper towels
Before observing internal or external structures of the fetal pig, use your dissection manual, textbook, and dissection notebook to answer the pre-lab questions on the fetal pig. You may have to refer to more than one dissection manual to answer all the questions so trade and share with other dissection groups.
***Wear your lab apron and eye cover at all times. Watch your time and be sure to clean up all equipment and working area each day before leaving.
Day 1 – External Anatomy
- Obtain a fetal pig and rinse off the excess preservative by holding it under running water. Lay the pig on its side in the dissecting pan and locate dorsal, ventral,& lateral surfaces. Also locate the anterior and posterior ends.
- A fetal pig has not been born yet, but its approximate age since conception can be estimated by measuring its length. Measure your pig’s length from the tip of its snout to the base of its tail and record this on your hand-in. Use the length/age chart on this sheet or the inside cover of your dissection manual to determine the age of your fetal pig & record this.
- Examine the pig’s head. Locate the eyelids and the external ears or pinnae. Find the external nostrils.
- Study the pig’s appendages and examine the pig’s toes. Count and record the number of toes and the type of hoof the pig has.
- Locate the umbilical cord. With scissors, cut across the cord about 1 cm from the body. Examine the 3 openings in the umbilical cord. The largest is the umbilical vein, which carries blood from the placenta to the fetus. The two smaller openings are the umbilical arteries which carry blood from the fetus to the placenta.
- Lift the pig’s tail to find the anus. Study the ventral surface of the pig and note the tiny bumps called mammary papillary. These are present in both sexes. In the female these structures connect to the mammary glands.
- Determine the sex of your pig by locating the urogenital opening through which liquid wastes and reproductive cells pass. In the male, the opening is on the ventral surface of the pig just posterior to the umbilical cord. In the female, the opening is ventral to the anus. Record the sex of your pig.
- Carefully lay the pig on one side in your dissecting pan and cut away the skin from the side of the face and upper neck to expose the masseter muscle that works the jaw, lymph nodes, and salivary glands. Label these on your hand-in.
- With scissors, make a 3-cm incision in each corner of the pig’s mouth. Your incision should extend posteriorly through the jaw.
- Spread the jaws open and examine the tongue.
- Observe the palate on the roof of the mouth. The anterior part of the palate is the hard palate, while the posterior part is the soft palate.
- Locate the epiglottis, a cone-shaped structure at the back of the mouth. Above the epiglottis, find the round opening of the nasopharynx. This cavity carries air from the nostrils to the trachea, a large tube in the thoracic which supplies air to the lungs.
- Dorsal to the glottis, find the opening to the esophagus. Examine the tongue and note tiny projections called sensory papillae.
- Examine the teeth of the pig. Canine teeth are longer for tearing food, while incisor are shorter and used for biting. Pigs are omnivores, eating plants and animals.
- Label the drawing of the inside of the pig’s mouth.
- Clean up your materials and work area. Wrap the pig in damp paper towels and put it in a zip-lock plastic bag. Obtain a piece of masking tape and label your bag with your names. Return your lab equipment and pig to the supply cart and then thoroughly wash your hands with soap.
Day 2 Part A: The Incision
- Be sure to wear your lab apron and eye cover. Obtain your dissecting equipment and pig from the supply cart.
- Place the fetal pig ventral side up in the dissecting tray.
- Tie a string securely around a front limb. Run the string under the tray, pull it tight, and tie it to the other front limb. Repeat this procedure with the hind limbs to hold the legs apart so you can examine internal structures.
- Study the diagram below. The dashed lines numbered 1-5 show the first set of incisions that you will make. To find the exact location for the incision marked 2, press along the thorax with your fingers to find the lower edge of the ribs. This is where you will make incision 2.
- With scissors, make the incisions in order, beginning with 1. Be sure to keep the tips of your scissors pointed upward because a deep cut will destroy the organs below. Also, remember to cut away from yourself.
- After you have made your incisions through the body wall, you will see the peritoneum, a thin layer of tissue that lines the body cavity. Cut through the peritoneum along the incision lines.
- Spread the flaps of the body wall apart. Cut the umbilical vein which extends through the liver.
- Once the vein is cut, carefully pull the flap of skin, including the end of the umbilical cord between the hind legs. Your are now able to see the organs of the abdominal cavity.
If time remains continue with part B, the digestive tract. Otherwise, clean up and return your materials and pig as you did on day 1.
Part B: Digestive System
- Be sure you are wearing your lab apron and eye cover.
- Locate the diaphragm, a sheet of muscle that separates the abdominal cavity from the thoracic cavity. Find the most obvious structure in the abdominal cavity, the brownish-colored liver. Count the number of lobes.
- Find the tube-like esophagus which joins the mouth and the stomach. Food moves down the esophagus by muscular contractions after being softened by saliva in the mouth. Follow the esophagus and locate the soft, sac-like stomach beneath the liver.
- With scissors, cut along the outer curve of the stomach. Open the stomach and note the texture of its inner walls. These ridges inside the stomach are called rugae and increase the area for the release of digestive enzymes. The stomach may not be empty because fetal pigs swallow amniotic fluid.
- The pig has a digestive system which is classified as monogastric or nonruminant. Humans also have this type of digestive system. They have one stomach (mono=one, gastric=stomach). Locate the entrance to the stomach or esophageal area, the cardiac region which is largest, and the pyloric region where the stomach narrows to join to the small intestine.
- At the end of the stomach, there is a sphincter, or ring-shaped muscle to control food leaving the stomach and entering the duodenum. Locate the cardiac sphincter at the junction of the stomach and esophagus, and the pyloric sphincter at the junction of the stomach and small intestine. Fetal pigs receive their nourishment from their mother through the umbilical cord.
- Identify the first part of the small intestine, the U-shaped duodenum, which connects to the lower end of the stomach. Pancreatic juice, made by the pancreas, and bile, made by the liver and stored in the gall bladder, are add to food here to continue digestion.
- Study the rest of the small intestine. Notice that it is a coiled, narrow tube, held together by tissue called mesentery. The soupy, partly digested food that enters the small intestine from the stomach is called chyme.
- Carefully cut through the mesentery and uncoil the small intestine. Note and record its length in centimeters. The mid-section is called the jejunum, while the last section is called the ileum.
- With scissors, remove a 3-cm piece of the lower small intestine. Cut it open and rinse it out.
- Observe the inner surface of the small intestine. Run your finger along it and note its texture. Using a magnifying glass, examine the villi, the tiny projections that line the small intestine and increase the surface area for absorption.
- Follow the small intestine until it reaches the wider, looped large intestine. Cut the mesentery and unwind the large intestine or colon. Measure and record its length.
- At the junction of the large and small intestine, locate a blind pouch called the caecum. The caecum has no known function in the pig.
- Notice that the large intestine leads into the rectum, a tube that runs posteriorly along the dorsal body wall. The rectum carries wastes to the opening called the anus where they are eliminated.
- Locate the thin, white pancreas beneath the stomach and duodenum. Pancreatic juice flows through pancreatic ducts to the duodenum.
- Between the lobes of the liver, find the small, greenish-brown gall bladder. Locate the hepatic duct which carries bile from the liver to the gall bladder.
- Find the spleen, a long, reddish-brown organ wrapped around the stomach. The spleen filters out old red blood cells and produces new ones for the fetus.
- On the diagram on the back of day 2 hand-in, label the pig’s body organs.
Clean up your materials and work area. Wrap the pig in damp paper towels and put it in a zip-lock plastic bag. Return your lab equipment and pig to the supply cart and then thoroughly wash your hands with soap.
Day 3 Respiratory System
- Be sure to wear your lab apron and eye cover.
- Examine the diaphragm, a sheet of muscle that stretches across the abdominal cavity and separates it from the thoracic cavity where the lungs are located. The diaphragm isn’t used by the fetal pig because gas exchange occurs through the umbilical cord. The diaphragm in adult pigs moves up and down changing air pressure in the chest cavity causing air to move into and out of the lungs.
- In order to see the upper part of the respiratory system, you will need to extend cut #1 up under the pig’s throat and make to more lateral incisions in order to fold back the flaps of shin covering the throat.
- In the thoracic cavity, carefully separate the pericardium or sac surrounding the heart and the diaphragm from the body wall.
- Locate the two, spongy lungs that surround the heart. The tissue that covers and protects the lungs is called pleura. The lungs haven’t been used by the fetus so they have never contained air.
- Find the trachea, a large air tube that lies anterior to the lungs. The trachea is easy to identify because of the cartilaginous rings that help keep it form collapsing as the animal inhales and exhales.
- Notice that the trachea branches into each lung. These two tubes are called bronchial tubes. Inside the lungs these branch into smaller bronchioles that end with a grape-like cluster of air sacs or alveoli where oxygen and carbon dioxide are exchanged with capillaries.
- Lying ventral to the trachea or windpipe, locate the pinkish-brown, V-shaped structure called the thyroid gland. This gland secretes hormones that control metabolism.
- At the top, anterior end of the trachea, find the hard, light-colored larynx or voice box. This organ contains the vocal cords that enable the animal to produce sound.
- Locate the epiglottis at the top of the trachea. This flap of skin closes over the trachea whenever you swallow. Find the area called the pharynx at the back of the nasal cavity. Air enters an adult pig through the mouth or nose before passing through the pharynx and down the trachea to the lungs.
- Label the diagram of the respiratory system on your day 3 hand-in.
Clean up your materials and work area. Wrap the pig in damp paper towels and put it in a zip-lock plastic bag. Return your lab equipment and pig to the supply cart and then thoroughly wash your hands with soap.
Day 4 Circulatory System
- Be sure to wear your lab apron and eye cover.
- Locate the heart. It is covered by a thin tissue called the pericardium. Remove this membrane to study the heart.
- Pigs, like all mammals, have four-chambered hearts. The right side of the heart pumps blood to the lungs, while the left side of the heart pumps blood to all other parts of the body. Locate the right and left sides of the heart.
- Each side of the heart has an upper and a lower chamber. Upper chambers are called atria and receive blood, while lower chambers are called ventricles and pump blood out of the heart. Locate the right and left atria and ventricle.
- Notice that the surface of the heart is covered with blood vessels. These are part of the coronary circulation, a set of arteries and veins whose only job is to nourish the heart tissue. Blockage in these vessels causes heart attacks.
- Anterior to the heart, locate another large vein that enters the right atrium. This vein, the anterior vena cava, brings blood to the right atrium from the anterior part of the body.
- Now lift the heart to view its dorsal surface. Observe the posterior vena cava that carries blood from the posterior part of the body and empties it into the right atrium.
- Find the pulmonary artery which leaves the right ventricle. After birth, this vessel carries blood to the lungs. However, in a fetus, a shunt called the ductus arteriosus allows fetal blood to bypass the lungs and go directly to the aorta, the largest artery of the body.
- Locate the pulmonary veins that enter the left atrium. After birth, these vessels carry oxygenated blood from the lungs to the heart.
- Identify the aorta, a large artery that transports blood from the left ventricle. Many arteries that carry blood throughout the body branch off of the
- Remove the heart by severing the blood vessels attached to it.
- Hold the dorsal and ventral surfaces of the heart with your thumb and forefinger and rest the ventricles on your dissecting tray. With a scalpel, cut the heart into dorsal and ventral halves. Caution: The scalpel is very sharp. Use it carefully and always cut away from yourself.
- Remove any material inside the heart and expose the walls of the atria and the ventricles.
- Study the internal features of these chambers and note where vessels leave or enter each chamber. Locate the valves between each atrium and ventricle. These structures prevent blood from flowing backward in the heart.
- Label the fetal pig heart diagram on your day 4 hand-in.
Clean up your materials and work area. Wrap the pig in damp paper towels and put it in a zip-lock plastic bag. Return your lab equipment and pig to the supply cart and then thoroughly wash your hands with soap.
Day 5 Urogenital System
- Be sure to wear your lab apron and eye cover.
- Remove the digestive organs to study the excretory and reproductive organs that make up the urogenital system.
- Locate the large, bean-shaped kidneys lying against the dorsal body wall. Notice that they are covered by the peritoneum. Kidneys filter wastes from blood.
- Find the ureters, tubes which extend from the kidneys to the bag-like urinary bladder. The urinary bladder lies between the umbilical arteries and temporarily stores liquid wastes filtered from the blood.
- Lift the urinary bladder to find the urethra, the tube which carries urine out of the body. Follow the urethra to the urogenital opening on the outside of the pig’s body.
- Make sure that incision #6 extends all the way to the anus but be careful to not cut too deep and damage the internal organs.
- Follow the directions below for locating the excretory and reproductive organs in either a male or female pig. When you finish observing the organs in a pig of one sex, exchange specimens with another classmate to view the organs in a pig of the opposite sex.
- In the male pig, locate the two scrotal sacs at the posterior end of the pig. If the pig is in the later stages of development, you will find a testis in each sac. If the pig is in an early stage of development, the oval-shaped testes will be in the abdominal cavity. These testes have not yet descended into the scrotal sacs.
- On each testis, find the coiled epididymis. Sperm cells produced in the testis pass through the epididymis and into a tube called the vas deferens. This tube crosses over a ureter and enters the urethra.
- Follow the urethra to the penis, a muscular tube lying just below the skin posterior to the umbilical cord. In mammals, the penis is the organ that transfers sperm.
- Label the diagram of the male urogenital system on your day 5 hand-in.
- In the female pig, find the two bean-shaped ovaries at the posterior end of the abdominal cavity. Observe the coiled Fallopian tube attached to each ovary, which carries eggs from the ovary.
- Follow the Fallopian tube to the uterus. The uterus is dorsal to the urinary bladder and the urethra.
- Trace the uterus to a muscular tube called the vagina. The vagina will appear as a continuation of the uterus. Sperm from the male are deposited into this organ during mating. The vagina and the urethra open into a common area called the urogenital sinus. This cavity opens to the outside at the urogenital opening.
- Label the diagram of the female urogenital system on your day 5 hand-in.
When you have completed your study of the urogenital system of both sexes, then clean up your materials and work area. Wrap the pig in damp paper towels and put it in a zip-lock plastic bag. Return your lab equipment and pig to the supply cart and then thoroughly wash your hands with soap.
Day 6 Nervous System
- Be sure to wear your lab apron and eye cover.
- With the pig dorsal side down, open both thoracic and abdominal flaps and locate the spinal column.
- Select a site along the spine and remove any organs blocking your view of the spine. Using a scalpel, expose the spine and locate any emerging nerves. Trace one as far as you can into the body.
- Place the pig dorsal side up in your dissecting tray. In the thoracic region, remove the skin and muscle to expose 10mm of the vertebral column.
- Using forceps to grip the spine and scissors to cut, open the vertebral canal by cutting off the vertebral arch. Note the dura mater or outermost covering of the brain & spinal cord.
- Make a second cut on the other side of this vertebrae, and fold the spine section upward so you can view the cross-section. Locate the white and gray matter, dorsal and ventral root, central canal, and a dorsal root ganglion.
- With the dorsal side of the pig up, remove the skin from the entire skull.
- Cut through the skull near the center being careful not to break the meninges or membranes covering and protecting the brain.
- After the skull is open, chip away the pieces but do not use the scalpel blade for chipping.
- When the brain is completely exposed, locate the 2 large hemispheres called the cerebrum. Fissures indenting the surface of the cerebrum are called sulci (sulcus, singular). Gyri (gyrus, singular) are ridges projecting outward from the surface.
- Locate the longitudinal fissure or indention that runs laterally between the right and left cerebral hemispheres. The olfactory lobes that control smell are at the front of the cerebrum. The cerebrum controls thinking, senses, etc.
- Posterior to the cerebrum is the cerebellum. Locate the cerebellum and the transverse fissure that separates it from the cerebrum. The cerebellum consists of 2 lateral hemispheres and is involved with the control of muscles and coordination.
- Find the fissure between the right and left cerebellum hemispheres called the vermis.
- Carefully remove the brain from the skull in order to locate the hind section of the brain known as the medulla oblongata. The medulla connects the brain to the spinal cord and controls all vital functions of the body such as heart beat and breathing.
- Label the diagrams of the brain and spinal cord on your day 6 hand-in.